1. Field of the Invention
The present invention relates to a process and a device for bit-rate reduction for the recording of images on a video recorder.
It applies in particular to the digital recording of an image but also to the transmission and storage of images.
2. Discussion of the Background
For the high definition digital television studios of the future it would appear to be essential to be able to record the high definition television signal in digital form in order to preserve the quality of the images in the course of the multiple reading recording operations which take place while editing recording tapes. The known analog high definition video recorders use magnetic tape reels and this recording medium could also be envisaged for digital video recorders. However, to allow convenient and reliable use, cassette-based video recorders would be better adapted. In fact, current technology would appear to be unsuited to these choices since, to obtain the high data throughput demanded by such a recording, it would be necessary to use a very large number, 8 or 16, of reading heads, and a magnetic tape and a recording method of much higher performance than those currently available. Other problems related to recording on cassette are related to the fact that the time available on a tape should exceed at least 60 minutes and that the recording of the sound should be carried out on at least four digital audio channels. To meet these requirements, digital solutions with signal bit-rate compression may naturally be envisaged while conforming with the main constraints related to the use of a digital video recorder. A first constraint is that a fixed bit rate per frame or per image must be employed so as to allow image-by-image editing and random access for the images on the recording tape. Random access to the images requires an image or a frame always to commence exactly synchronously with the instant of synchronization recorded on the tape, or otherwise searching and positioning will be impossible. Image-by-image editing requires the repositioned image to occupy the same space on the tape as the new image and also the images to be truly independent of one another. In accordance with a second constraint, the coding used must allow image reading in special modes such as "slow motion", "speeded up" and the "freeze-frame advance" mode. In the "slow motion" modes, reconstruction of the images must be perfect and in the "speeded up" modes it must be possible to recognize the images. It should be appreciated that for the currently existing cassette-based video recorders, in the "slow motion" modes the information read contains few errors on account of the techniques known as "overscanning" and "tracking", slow motion can be obtained by repeating frames or by interpolation. In the "fast forward" modes, however, owing to the position of the reading heads relative to the tape, only a part of the information is read (the remainder is interpolated). Moreover, for one and the same image, some lines originate from the image at the instant t and others from the image at instants t+1, t+2, . . . , depending on the reading speed. Finally, in accordance with a third constraint, for professional applications it is imperative to guarantee quality of reconstruction without visible degradation of the image even after a coding/decoding series.
Most of the existing bit-rate reduction techniques were developed for transmission applications. They are generally based on splitting the image by cosine (or sub-band) transform followed by variable-length coding. The mean bit rate is fixed, although the bit rate can vary from one image to another. Moreover, to obtain high compression rates these techniques employ inter-image coding with motion compensation so that the images depend on the previous images.
For recording applications, the main problem is of keeping a fixed bit rate per image or image zone (per sector recorded for example) while using techniques adaptable to the contents of the image, these techniques being the only ones which provide satisfactory results. For this purpose, there are two ways of solving the problem, either by performing two passes, a first pass to obtain the binary cost of the image which allows adjustment of the parameters in order to code the image to the greatest possible extent within the available bit rate or with a pre-analysis of the image which allows the available bit rate to be allocated, as a function of the contents of each block of the image. These methods are associated with the conventional techniques used in transmission by cosine transform (or sub-band) splitting, quantization and variable-length coding or with novel methods developed for this particular application. These methods are for example described:
in the article by N. ENDOH entitled "Experimental digital VCR with New DCT-Based Bit-Rate Reduction and Channel Coding" published in the SMPTE Journal of July 1992, on pages 475 to 480
in the article by P. KAUFF entitled "A DCT Coding Scheme for Digital HDTV Recording" published in the 4th International Workshop on High Definition Television which took place in Turin in Italy in 1991
in the article by T. KONDO et al entitled "Adaptative Dynamic Range Coding Scheme for Future HDTV Digital VTR" published in the 4th International Workshop on High Definition Television in Turin in Italy in 1991
in the article by K. ONISHI et al entitled "An Experimental Home-Use Digital VCR with Three Dimensional DCT and Superimposed Error Correction Coding" published in IEEE Transactions on Consumer Electronics, Vol. 37, No. 3, August 1991
in the article by J. MAX entitled "Quantizing for Minimum Distorsion" published in the IRE journal Transaction Inform Theory, Vol. IT-6, 7-12 January 1960
in the article by P. NOLL and R. ZELINSKI entitled "Comments on: Quantizing Characteristics for Signals Having Laplacian Amplitude Probability Density Function" published in the IEEE article Transactions on Communications, Vol. COM-27, No. 8, August 1979.
In the article by N. ENDOH the coding technique is termed intra-image (that is to say independent from one image to another) with image splitting by cosine transform, quantization and then variable-length coding, the fixed bit rate is obtained by pre-analysis of the image. The latter consists of a calculation of energy per block which makes it possible to allocate the bit rate locally as a function of the difficulty of coding the blocks, and of a calculation of the global energy of the image which makes it possible to allocate the total bit rate available to code the image. However, this technique which was developed for a data formatting other than that used in current video recorders cannot solve a priori the problem of the "speeded up" modes.
In the article by P. KAUFF the method used is based on the same principle as before with a pre-analysis but also with a technique which makes it possible to solve in part the problem of the "speeded up" modes, the largest coefficients of the cosine transform not being coded with variable-length codes but inserted regularly on to the tape and on a fixed number of bits. Moreover, a choice is made between the intra-frame or intra-image modes. Nevertheless, certain problems remain unsolved, in particular as regards the solutions which have to be taken secondarily when the bit rate is exceeded. Furthermore, the result obtained in terms of image quality in the "speeded up" mode would appear to be uncertain. Moreover, this method necessitates the development of a novel video recorder which takes account of the fact that the formatting of the data on the tape is modified.
In the article by T. KONDO the method used is different since in order to obtain a bit rate which is fixed per image period that is to say over two images, the technique does not use variable-length coding, the binary cost of which is very difficult to foresee. The technique employed consists in chopping the image into blocks with 3 dimensions, with 6 lines and 6 pixels for the two consecutive images, and in then calculating the dynamic range inside each block by pre-analysis so as to allocate locally the bit rate necessary for each block. The sum of the dynamic ranges over the image then makes it possible to allocate the total bit rate available to the whole image. With this method the bit rate is fixed for 2 images. However, it does not allow the fast forward modes and editing is limited to sets of two images by two images. The results obtained would therefore appear to be inadequate for the production of a professional video recorder.
In the article by K. ONISHI a three-dimensional cosine transform involving four images, followed by a splitting into four frequency bands are employed. A first pass determines the cost of each block and the quantization is adapted in accordance with the available bit rate. However, the article offers no solution in respect of the speeded up modes and editing is limited to four images. Also, the signal-to-noise ratio is inadequate to allow production of professional video recorders.